Visual information processing in the primate cerebral cortex begins in the striate cortex and continues into extrastriate areas. One of these areas, the middle temporal area (MT), has a high proportion of cells that respond to stimuli moving within a restricted range of directions and velocities. Our work on this area in preceding years has concentrated on the visual and visual-motor properties of cells within this area including their role in initiation of slow pursuit eye movements made to moving visual targets. This year we have investigated the contribution of these cells to the perception of visual motion by using a visual illusion of motion. A sequence of stationary lights separated by appropriate spatial and temporal intervals gives rise to a vivid perception of motion, usually referred to as the phi-phenomena. We compared the threshold for a clear impression of motion in humans with the threshold at which directional selectivity became evident in striate cortex and MT cells. When we compared the human performance to that of a composite of the physiological responses, we found that human psychophysical performance corresponded to the best physiological response at all speeds. Which area, MT or striate cortex, corresponded more closely to the psychophysics, however, varied with the speed. At low speeds (2 degrees/sec), humans perceive motion for temporal intervals which in general did not sustain directional selective response in MT neurons but did in striate neurons. On the other hand, at high speeds (32 degrees/sec), perceived motion was more comparable to the response of MT cells. The results support the idea that directional selective cortical neurons are possible substrates for perception of apparent motion.